[clang] [Clang][Concepts] Fix a constraint comparison regression for out-of-line ClassTemplateDecls (PR #102587)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

Note that @jcsxky has a similar patch https://github.com/llvm/llvm-project/pull/102554 that fixes the same thing. I don't intend to contend against him, and admittedly, that patch works.

But I don't think that approach is so reasonable because it is hacky in that `ForConstraintInstantiation` is disabled in the presence of `ClassTemplateDecl`s, which seemingly does a superfluous job that `getTemplateInstantiationArgs()` should do and blurs the meaning of the flag.

That would also leave us a concern about whether we should change all the other lines where `ForConstraintInstantiation` was true to the same pattern in `SubstituteConstraintExpressionWithoutSatisfaction`.

https://github.com/llvm/llvm-project/pull/102587